A conventional subsea wellhead assembly includes a wellhead housing which supports one or more casing hangers located at upper ends of strings of casing extending into the well. A tubing hanger lands in the wellhead housing above the casing hanger and supports a string of production tubing that extends through the smallest diameter casing. The tubing hanger has a production bore which is offset slightly from the longitudinal axis. An annulus bore also extends through the tubing hanger, parallel to and offset from the axis, for communicating the tubing annulus to above the tubing hanger. The annulus bore is needed during installation of the tubing hanger and tubing to establish circulation down the tubing and back up the annulus. After the well has been completed, a removable plug is installed in the annulus bore, then a production tree is mounted to the wellhead housing. Access through the production tree to the tubing may be made for various workover operations that are needed.
In the last few years, operators have begun installing a different type of wellhead assembly, referred to generally as a horizontal tree. In a horizontal tree, the tubing hanger lands in the tree, not in the wellhead housing located below the tree. The tubing hanger has a lateral flow passage extending from its vertical flow passage. The lateral flow passage registers with a lateral flow passage extending through a sidewall of the tree. Gallery seals are employed to seal the junction between the lateral production passages. The gallery seals comprise seal rings which are coaxial with the vertical axis, with one of the seals located above the lateral passage and the other located below. The lower seal necessarily will be of a smaller diameter than the upper seal in order to provide clearances for installation.
With the horizontal tree, a tubing hanger can be pulled through the horizontal tree without removing the tree. This cannot be done with a conventional tree. While this is an advantage, one disadvantage is the horizontal tree tubing hanger has inadequate room to utilize a vertical annulus passage extending through the a tubing hanger as with a conventional tubing hanger. as Instead, tubing annulus communication is accomplished generally by utilizing a bypass passage through the tree from below the tubing hanger and back into the tree above the tubing hanger. While a bypass passage is workable, it relies on a valve on the exterior for closing the annulus. Some operators believe that a removable plug installed within an annulus passage in a tubing hanger is safer than a valve.
Another disadvantage of a typical horizontal tree tubing hanger has to do with the need to communicate auxiliary fluid to downhole equipment. For example, downhole safety valves are used in a tubing string at some distance below the surface. A safety valve remains open so long as it is supplied with hydraulic fluid pressure. In the absence of fluid pressure, it will close. Consequently, if the production wellhead assembly is severely damaged, the well would be held under control through the safety valve. In conventional tree tubing hangers, passages are drilled through the tubing hanger from the upper end to the lower end. The upper ends of the hydraulic passages have connectors which mate with connectors on the tree to supply hydraulic fluid. In the horizontal tree, however, this cannot occur because the tubing hanger lands within the tree, not in the wellhead housing below.
Some manufacturers have drilled ports through the sidewall of the tree to communicate with hydraulic passages drilled within the tubing hanger. These manufacturers have employed gallery type seals to seal the junctions of the ports. This again requires a reduction in inner diameter of the bore of the tree. There may be several ports for auxiliary fluid passages, requiring several sets of gallery seals. U.S. Pat. Nos. 5,465,794 and 5,555,935 show ports on the exterior of a tubing hanger that do not requires gallery seals. These ports locate on a spherical surface formed on the tubing hanger and in the bore of the tree.